Add "mock" compilation feature to dart2js and use it to improve test coverage.

dart/frog/scripts/buildbot_annotated_steps.py

 #!/usr/bin/python
 
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Dart frog buildbot steps
 
 Runs tests for the frog compiler (running on the vm or the self-hosting version)
 """
@@ skipping 162 matching lines @@
     # TODO(jmesserly): make DumpRenderTree more like other browser tests, so
     # we don't have this translation step. See dartbug.com/1158.
     # Ideally we can run most Chrome tests in DumpRenderTree because it's more
     # debuggable, but still have some tests run the full browser.
     # Also: we don't have DumpRenderTree on Windows yet
     # TODO(efortuna): Move Mac back to DumpRenderTree when we have a more stable
     # solution for DRT. Right now DRT is flakier than regular Chrome for the
     # isolate tests, so we're switching to use Chrome in the short term.
     if browser == 'chrome' and system == 'linux':
       TestStep('browser', mode, system, 'frogium', tests, flags)
-      TestStep('browser', mode, system, 'legium', tests, flags)
+      TestStep('browser', mode, system, 'legium', [], flags)
+      TestStep('browser', mode, system, 'legium_extra',
+               ['leg_only', 'frog_native'], flags)
     else:
       additional_flags = ['--browser=' + browser]
       if system.startswith('win') and browser == 'ie':
         # There should not be more than one InternetExplorerDriver instance
         # running at a time. For details, see
         # http://code.google.com/p/selenium/wiki/InternetExplorerDriver.
         additional_flags += ['-j1']
       TestStep(browser, mode, system, 'webdriver', tests,
           flags + additional_flags)
 
@@ skipping 66 matching lines @@
   if status != 0:
     print '@@@STEP_FAILURE@@@'
 
   if component == 'frogium':
     CleanUpTemporaryFiles(system, browser)
   return status
 
 
 if __name__ == '__main__':
   sys.exit(main())